High voltage current source with short circuit protection

ABSTRACT

A current source generating a charging current and a supply voltage by charging a capacitor with the charging current. The current source has a conversion circuit converting a line voltage into a second voltage, a current generation circuit generating the charging current based on the second voltage, and a control circuit controlling the current generation circuit based on the supply voltage. The charging current is controlled to be at a first current value when the supply voltage is lower than a first threshold voltage, the charging current is controlled to be at a second current value when the supply voltage is higher than a second threshold voltage. The first threshold voltage is lower than the second threshold voltage, and the first current value is lower than the second current value.

CROSS REFERENCE

This application claims the benefit of CN application No.201410699713.1, filed on Nov. 27, 2014, and incorporated herein byreference.

FIELD OF THE INVENTION

This disclosure generally relates to electronic circuits, and moreparticularly but not exclusively to high voltage current source withshort circuit protection.

BACKGROUND OF THE INVENTION

High voltage current sources are required in almost all AC-DCconverters. A high voltage current source generates a charging currentby converting a line voltage that is rectified from an AC voltage, suchas a 220V AC voltage. A voltage is then generated by charging acapacitor with the charging current, and is further provided to supply acontrol circuit of a converter during the startup period.

The charging current of the traditional high voltage current source issimply converted from the line voltage, thus, the power dissipation ishigh and the reliability is bad.

Accordingly, an improved high voltage current source having low powerdissipation and good reliability is required.

SUMMARY

Embodiments of the present invention are directed to a novel currentsource having an input and an output. The current source receives a linevoltage at the input and provides a charging current at the output. Thecharging current is provided to charge a capacitor to generate a supplyvoltage at the output of the current source. The current sourcecomprises a conversion circuit, a current generation circuit, and acontrol circuit. The conversion circuit converts the line voltage into asecond voltage, which is lower than the line voltage. The currentgeneration circuit receives the second voltage and generates thecharging current. The control circuit is coupled to the currentgeneration circuit. The control circuit receives the supply voltage, andcontrols the current generation circuit based on the supply voltage. Thecontrol circuit controls the current generation circuit such that thecharging current is at a first current value when the supply voltage islower than a first threshold voltage and at a second current value whenthe supply voltage is higher than a second threshold voltage. The firstthreshold voltage is lower than the second threshold voltage and thefirst current value is lower than the second current value.

Embodiments of the present invention are also directed to a novel methodfor supplying power to an isolation converter with short circuitprotection. The method comprises: rectifying an AC voltage into a linevoltage; converting the line voltage into a second voltage that is lowerthan the line voltage; generating a first current based on the secondvoltage and generating a second current based on the first current; andgenerating a charging current based on the second current through acurrent mirror. The charging current is provided to generate a supplyvoltage by charging a capacitor. The second current is controlled to beat a first current value when the supply voltage is lower than a firstthreshold voltage, and to be at a second current value when the supplyvoltage is higher than a second threshold voltage. The second thresholdvoltage is higher than the first threshold voltage and the secondcurrent value is higher than the first current value.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of various embodiments of the presentinvention can best be understood when read in conjunction with thefollowing drawings, in which the features are not necessarily drawn toscale but rather are drawn as to best illustrate the pertinent features.

FIG. 1 illustrates a schematic diagram of an isolation conversion system100 in accordance with an embodiment of the present invention.

FIG. 2 illustrates a block diagram of a current source 200 in accordancewith an embodiment of the present invention.

FIG. 3 illustrates a schematic diagram of a current source 300 inaccordance with an embodiment of the present invention.

FIG. 4 illustrates a workflow of method 400 of supplying power to anisolation converter with short circuit protection in accordance with anembodiment of the present invention.

The use of the same reference label in different drawings indicates thesame or like components or structures with substantially the samefunctions for the sake of simplicity.

DETAILED DESCRIPTION

Various embodiments of the present invention will now be described. Inthe following description, some specific details, such as examplecircuits and example values for these circuit components, are includedto provide a thorough understanding of embodiments. One skilled in therelevant art will recognize, however, that the present invention can bepracticed without one or more specific details, or with other methods,components, materials, etc. In other instances, well-known structures,materials, processes or operations are not shown or described in detailto avoid obscuring aspects of the present invention.

Throughout the specification and claims, the term “coupled,” as usedherein, is defined as directly or indirectly connected in an electricalor non-electrical manner. The terms “a,” “an,” and “the” include pluralreference, and the term “in” includes “in” and “on”. The phrase “in oneembodiment,” as used herein does not necessarily refer to the sameembodiment, although it may. The term “or” is an inclusive “or”operator, and is equivalent to the term “and/or” herein, unless thecontext clearly dictates otherwise. The term “based on” is not exclusiveand allows for being based on additional factors not described, unlessthe context clearly dictates otherwise. The term “circuit” means atleast either a single component or a multiplicity of components, eitheractive and/or passive, that are coupled together to provide a desiredfunction. The term “signal” means at least one current, voltage, charge,temperature, data, or other signal. Where either a field effecttransistor (“FET”) or a bipolar junction transistor (“BJT”) may beemployed as an embodiment of a transistor, the scope of the words“gate”, “drain”, and “source” includes “base”, “collector”, and“emitter”, respectively, and vice versa. Those skilled in the art shouldunderstand that the meanings of the terms identified above do notnecessarily limit the terms, but merely provide illustrative examplesfor the terms.

The current source according to an embodiment of the present inventiongenerates a charging current from a line voltage, wherein the currentsource has short circuit protection function and low power consumption.

FIG. 1 illustrates a schematic diagram of an isolation conversion system100 in accordance with an embodiment of the present invention. Isolationconversion system 100 comprises a rectifier circuit 12, an isolationconverter, a controller 10 and a current source 11. Isolation conversionsystem 100 is configured to convert an AC input voltage Vac, such as anAC voltage with a value of 85˜265 volts, into a DC output voltage Voutto supply a load. In the shown embodiment, a flyback converter utilizedin the isolation conversion system 100 comprises a primary winding L1, amain secondary winding L2 and an auxiliary secondary winding L3, a mainswitch K and an output filter circuit 13. An induced voltage isgenerated at the secondary winding L2 by switching the main switch K onand off, under the control of the controller 10 that is coupled to acontrol end of the main switch K. The induced voltage is then convertedinto the output voltage Vout through the filter circuit 13. It should beknown that the current source in the embodiment of FIG. 1 may be alsoapplied in another isolation conversion system comprising any othersuitable type of converter besides the flyback converter, or in anon-isolation conversion system. Rectifier circuit 12 has an inputconfigured to receive the AC input voltage Vac, and an output configuredto provide a line voltage HV. The isolation converter has an input andan output, wherein the input of the isolation converter is coupled tothe output of the rectifier circuit 12, and the output of the isolationconverter is coupled to the load. The isolation converter comprises aprimary winding L1, a main secondary winding L2 and an auxiliarysecondary winding L3. The main secondary winding L2 is coupled to theoutput of the isolation converter, and the auxiliary secondary windingL3 is configured to provide a supply voltage VCC to the controller 10during the normal operation of the isolation conversion system 100.

During the normal operation of the isolation conversion system 100,supply voltage VCC supplying power to the controller 10 and contributingto generate a feedback signal COMP is generated by switching the mainswitch K on and off. As shown in FIG. 1, the auxiliary secondary windingL3 is configured to generate the induced voltage by switching the mainswitch K on and off, wherein the induced voltage is provided to generatesupply voltage VCC.

During the startup period of the isolation conversion system 100, theswitching action of the main switch K is not in the normal work state,and the induced voltage provided by the auxiliary secondary winding L3is not large enough to supply the controller 10. Thus, the currentsource 11 is used to supply power to the controller 10 during thestartup period of the isolation conversion system 100. Current source 11is configured to generate a charging current Ic based on the linevoltage HV. And by charging a capacitor with charging current Ic, supplyvoltage VCC is generated. In this embodiment, current source 11 has ashort circuit protection function. That is, charging current Icgenerated by the current source 11 is small when supply voltage VCCapproximates a ground voltage, to avoid heat accumulation. And chargingcurrent Ic is increased when supply voltage VCC increases to apredetermined value. And further, when the isolation conversion system100 operates in the normal state, that is, when the induced voltagegenerated by the auxiliary winding L3 is large enough, current source 11is shut down to decrease charging current Ic to zero.

FIG. 2 illustrates a block diagram of a current source 200 in accordancewith an embodiment of the present invention. Current source 200 isconfigured to generate a supply voltage VCC by charging a capacitor C1with a charging current Ic that is generated based on a line voltage HV.

Current source 200 comprises a conversion circuit 21, a currentgeneration circuit 22 and a control circuit 23. Conversion circuit 21 isconfigured to convert the line voltage HV into a second voltage V2 thatis smaller than the line voltage HV, for example, to convert a linevoltage of 300 volts into a second voltage of 70 volts. Wherein the linevoltage HV and the second voltage V2 are both DC voltages.

Current generation circuit 22 is coupled to conversion circuit 21 andcontrol circuit 23. Current generation circuit 22 is configured toreceive the second voltage V2 and to generate a charging current Ic atan output of current generation circuit 22 based on the second voltageV2. Charging current Ic charges the capacitor C1 to generate a supplyvoltage VCC.

Control circuit 23 is coupled to current generation circuit 22, and isconfigured to receive supply voltage VCC and an external control signalSHUT. Control circuit 23 is configured to control the current generationcircuit 22 based on supply voltage VCC and the external control signalSHUT. When supply voltage VCC is lower than a first threshold voltage,control circuit 23 controls the current generation circuit 22 so thatcharging current Ic generated by current generation circuit 22 is at afirst current value, wherein the first current value is relatively smallor may be expressed as: Ic<Ith1. When supply voltage VCC is higher thana second threshold voltage, control circuit 23 controls currentgeneration circuit 22 so that charging current Ic generated by currentgeneration circuit 22 is at a second current value, wherein the secondcurrent value is relatively large or may be expressed as: Ic>lth2.Wherein the second threshold voltage is higher than the first thresholdvoltage, and the second current value is higher than the first currentvalue, expressed as: Ith2>Ith1. When the external control signal SHUT isin a predetermined state and supply voltage VCC is higher than athreshold voltage, control circuit 23 controls current generationcircuit 22 to enter into an OFF state to decrease charging current Ic tozero. In one embodiment, when supply voltage VCC is higher than a thirdthreshold voltage, the external control signal SHUT is in thepredetermined state. In another embodiment, when an output voltage of aconverter is higher than a threshold voltage, the external controlsignal SHUT is in the predetermined state. It should be known that, “thefirst threshold voltage”, “the second threshold voltage” and “the thirdthreshold voltage” may not indicate real signals but some abstractvalues, for example, “the first threshold voltage” may be related to thegate voltage of transistor MN3 which ensures transistor MN3 is in an OFFstate, “the second threshold voltage” may be related to the gate voltageof MOSFET MN3 which ensures transistor MN3 in an ON state. The term“external signal” herein only indicates that the signal can be generatedoutside of current source 200, without the limitation of outside of anintegrated circuit, outside of the control circuit or outside of theconverter.

FIG. 3 illustrates a schematic diagram of a current source 300 inaccordance with an embodiment of the present invention. Current source300 comprises a conversion circuit 31, a current generation circuit anda control circuit 35. Current generation circuit comprises a firstcurrent generation circuit 32, a second current generation circuit 33and a current mirror 34.

Convertion circuit 31 is configured to convert a line voltage HV into asecond voltage V2 that is lower than the line voltage HV. Conversioncircuit 31 comprises a Junction Field Effect Transistor (JFET) J1. Afirst end of JFET J1 is configured as an input of conversion circuit 31and is coupled to a line voltage node to receive the line voltage HV, asecond end of JFT J1 is configured as an output of conversion circuit 31and is coupled to the current generation circuit to provide the secondvoltage V2, and the control end of JFET J1 is coupled to the ground GND.In another embodiment, the control end of JFET J1 may be coupled toother nodes, such as a node of control circuit 35. In one embodiment,JFET J1 is a high voltage JFET having a breakdown voltage of about 700V.In one embodiment, JFET J1 converts a 300V line voltage into a 30Vsecond voltage V2.

First current generation circuit 32 is configured to generate a firstcurrent I1 based on the second voltage V2. The first current generationcircuit 32 comprises transistors J3, MPS, MN1, MP1, MP2 and a resistorR3, wherein transistor MP1 and transistor MP2 together form a currentmirror. Transistor MP1 as shown is a Metal Oxide Semiconductor FieldEffect Transistor (MOSFET), wherein the source of transistor MP1 iscoupled to conversion circuit 31 to receive the second voltage V2, thedrain of transistor MP1 is coupled to transistor J3, the gate and thedrain of transistor MP1 are shorted together. Transistor J3 is a JFET,wherein the source of transistor J3 is coupled to transistor MP1, thedrain of transistor J3 is coupled to a first end of resistor R3, and thecontrol end of transistor J3 is coupled to the ground GND. Resistor R3is coupled between the drain of transistor J3 and the source oftransistor MP5. Thus, resistor R3, transistor J3 and transistor MP1 arecoupled in series. Transistor MN1 and transistor MP5 are configured toselectively provide a first conducting path and a second conducting pathof first current generation 32. Thus, transistor MN1 and transistor MP5may be respectively referred to as a first conducting switch and asecond conducting switch of first current generation 32. The firstconducting switch MN1 and the second conducting switch MP5 are coupledin parallel. The drain of transistor MP5 is coupled to the ground GND,the control end of transistor MP5 is coupled to control circuit 35 andtransistor MP5 is thus controlled by control circuit 35. When supplyvoltage VCC is about zero, such as when supply voltage VCC is lower thana threshold voltage, transistor MP5 is in an ON state and transistor MN1is in an OFF state. When supply voltage VCC increases and reaches athreshold voltage, transistor MN1 is turned on and transistor MP5 isturned off. Transistor MN1 is coupled between a second end of resistorR3 and the ground GND, the control end of transistor MN1 is coupled tocontrol circuit 35. When supply voltage VCC continues to increase and anexternal control signal SHUT is in a predetermined state, such as LOWstate, transistor MN1 is also turned off. The source of transistor MP2is coupled to the output of conversion circuit 31, the drain oftransistor MP2 is coupled to the second current generation circuit 33,and the gate of transistor MP2 is coupled to the gate of transistor MP1.Transistor J3 converts the second voltage V2 into a voltage which islower than the second voltage V2. When either transistor MP5 ortransistor MN1 is turned on, the first transistor (transistor MP1) ofcurrent mirror, JFET J3, resistor R3 and either the first conductingswitch MN1 or the second conducting switch MP5 together form a currentpath to generate a base current I10. In one embodiment, the value ofbase current I10 is determined by the second voltage V2, the forwardvoltage of transistor J3 and the resistance of resistor R3. Currentmirror consisting of the first transistor MP1 and the second transistorMP2 mirrors base current I10 and outputs a first current I1 at an outputof the first current generation circuit 32, wherein the first current I1is proportional to the base current I10. In one embodiment, base currentI10 is smaller than the first current I1.

Second current generation circuit 33 is configured to generate a secondcurrent I2 based on the first current I1. Second current generationcircuit 33 comprises a bipolar QN1, a third transistor MN2, a fourthtransistor MN3, a fifth transistor MN4, a first current regulatingresistor R4 and a second current regulating resistor R5. The firstcurrent regulating resistor R4 and the second current regulatingresistor R5 are coupled in series. In detail, a second end of resistorR4 is coupled to a first end of resistor R5. The collector of bipolarQN1 is coupled to the output of first current generation circuit 32 toreceive first current I1, the emitter of bipolar QN1 is coupled to theground GND and the base of bipolar QN1 is coupled to a first end ofresistor R4. A second end of resistor R5 is coupled to the ground GND.Transistor MN2 is coupled between the ground GND and the output of thefirst current generation circuit 32 and the control end of transistorMN2 is coupled to control circuit 35. Under the control of controlcircuit 35, transistor MN2 is turned on and transistor MN4 is turned offwhen the external control signal SHUT is in a predetermined state.Transistor MN4 is coupled between resistor R4 and the output of secondcurrent generation circuit 33.

Current mirror 34 comprises transistors MP3 and MP4. Current mirror 34is configured to mirror and amplifier the second current I2 at a firstend of current mirror 34, and to provide a charging current Ic at asecond end of current mirror 34 to charge capacitor C1. Wherein the gateof transistor MP3 and the gate of transistor MP4 are shorted together,the source of transistor MP3 and the source of transistor MP4 areshorted together and configured to receive the second voltage V2, thegate and the drain of transistor MP3 are shorted together, and the drainof transistor MP3 is coupled to the output of the second currentgeneration circuit 33 for allowing the second current I2 to flow throughtransistor MP3. The drain of the transistor MP4 is configured as anoutput of the current mirror 34 as well as an output of the currentsource 300, and charging current Ic flows through the conducting pathformed by the source and the drain of transistor MP3. In one embodiment,transistor MP4 and transistor MP3 respectively comprise multiple MOSFETtransistor cells that are coupled in parallel and fabricated by the samemanufacturing process. In one embodiment, the number of the transistorcells of transistor MP4 is M times of that of transistor MP3 so thatcharging current Ic is M times of the second current I2, expressed as:Ic=M*I2. The startup time of the conversion system is controlled byadjusting charging current Ic.

Control circuit 35 is configured to control the second currentgeneration circuit 33 to output a small charging current Ic when supplyvoltage VCC is low, to output a large charging current Ic when supplyvoltage VCC is high, and to shut down current source 300 to decreasecharging current Ic to zero when the conversion system operates in thenormal state, such as when supply voltage VCC is higher than a thirdthreshold voltage and the external control signal SHUT is in apredetermined state, such as LOW state. Control circuit 35 comprises aJFET J2, a resistor R1, a resistor R2 and an invertor INT1. Wherein JFETJ2, resistor R1 and resistor R2 are coupled in series. The drain of JFETJ2 is coupled to supply voltage VCC and the resistor R2 is coupled theground GND. A first control signal is provided at the connection node oftransistor J2 and resistor R1 to control the first current generationcircuit 32 to provide a current during the startup period of the system.In one embodiment, transistor J2 is a JFET having a pinch-off voltage of8 volts, wherein the voltage at the source end (source voltage) oftransistor J2 varies with supply voltage VCC when supply voltage VCC islower than 8V, and the source voltage is fixed at 8V when supply voltageVCC is higher than 8V. The pinch-off voltage of transistor J2 decreasesthe power consumption of the conversion system when supply voltage VCCis high on the one hand, and guarantees the current supply and the shortcircuit protection when supply voltage VCC is relative low on the otherhand. In addition, a second control signal is provided at the connectionnode of resistors R1 and R2 to control the second current generationcircuit 33 so that the second current I2 is controlled. Thus, the firstcontrol signal and the second control signal increases or decreasesaccordingly with the increase or decrease of supply voltage VCC whensupply voltage VCC is low. An input of invertor INT1 is coupled to theexternal control signal SHUT and the first current generation circuit32, and an output of invertor INT1 is coupled to the second currentgeneration circuit 33. When the conversion system operates in the normalstate, such as when supply voltage VCC is higher than the thirdthreshold voltage, the external signal SHUT is in the predeterminedstate, for example, logic low, and the first current generation circuit32 is thus shut down. When the conversion system operates in the normalstate and the external control signal SHUT is in the predeterminedstate, transistor MN2 is turned on and transistor MN4 is turned offthrough invertor INT1 , thus shutting off the second current generationcircuit 33.

Supply voltage VCC is zero when the conversion system just starts up. Atthis time, the voltage at the connection node of transistor J2 andresistor R1 is zero, and P-type transistor MP5 is thus in the ON state.Consequently, transistors MP1 and J3, resistor R3 and transistor MP5form a current path to provide a base current I10 generated based on thesecond voltage V2, wherein the base current I10 may be regulated byadjusting the resistance of resistor R3. In one embodiment, transistorJ3 has a pinch-off voltage that clamps the source voltage of transistorJ3 at this pinch-off voltage. Thus, transistor J3 converts the secondvoltage V2 into a voltage V10 which is lower than the second voltage V2,and the base current is thus I10≈V10/R3. Take a 8V pinch-off voltage oftransistor J3 for example, the source voltage of transistor J3 isclamped at 8V, and accordingly so is the voltage V10. Transistors MP1and MP22 mirror the base current I10 and output the first current I1 atthe output of the first current generation circuit 32, wherein the firstcurrent I1 is proportional to the base current I10. The first current I1controls the voltage Vbe between the base and the emitter of bipolar QN1in the second current generation circuit 33, and the voltage Vbe isconfigured to control the second current I2.

When supply voltage VCC is low, the voltage at the connection node ofresistors R1 and R2 is low correspondingly. When the voltage at theconnection node is lower than a first threshold voltage, such as whenthe voltage at the connection node is lower than the on-thresholdvoltage Vgs of transistor MN3, expressed as:

${{VCC} < {\left( {1 + \frac{R\; 1}{R\; 2}} \right)*{Vgs}}},$transistor MN3 is in an OFF state, and correspondingly the secondcurrent I2 is:

${{I\; 2} = \frac{Vbe}{{R\; 4} + {R\; 5}}},$and charging current Ic is:

${Ic} = {M*{\frac{Vbe}{{R\; 4} + {R\; 5}}.}}$Resistor R5 may have a relatively high resistance so that the secondcurrent I2 as well as the corresponding charging current Ic iscontrolled to have a small value when supply voltage VCC is smaller thanthe first threshold voltage, such as

${\left( {1 + \frac{R\; 1}{R\; 2}} \right)*{Vgs}},$during the startup of the conversion system. In this way, when supplyvoltage VCC is close to zero, charging current Ic is relative small toprotect the system. The heat will increase sharply to destroy the systemwhen the output of the current source is shorted. The short circuitprotection during the startup of the conversion system is thus realized.In one embodiment, resistors R4 and R5 are both high-sheet polysiliconresistors, which have a temperature compensation coefficient thatensures the charging current immune to the temperature.

Supply voltage VCC increases gradually when being charged by chargingcurrent Ic. When supply voltage VCC rises beyond a second thresholdvoltage, such as:

${2*\left( {1 + \frac{R\; 1}{R\; 2}} \right)*{Vgs}},$transistor MN3 is fully turned on, thus shorting resistor R5. As aresult, the second current I2 increases to

${I\; 2} = \frac{Vbe}{R\; 4}$and charging current Ic increases to

${Ic} = {M*{\frac{Vbe}{R\; 4}.}}$Thus, charging current Ic can charge capacitor C1 quickly, supplyvoltage VCC raises fast and the charge efficiency is improved. Thethreshold voltage for short circuit protection may be regulated byadjusting the ratio of the resistance of resistor R1 to the resistanceof resistor R2. When supply voltage VCC is smaller than the thresholdvoltage for short circuit protection, control circuit 35 provides shortcircuit protection by clamping charging current Ic.

When supply voltage VCC is low, a logic circuit inside of a controllerof the conversion system is out of operation, the external control logicsignal and an output signal provided by invertor INT1 are both in a LOWstate, thus, N-type transistors MN1 and MN2 are both in the OFF state.With the increase of supply voltage VCC, the logic circuit inside of thecontroller starts to operate normally, the external control logic signalSHUT is in a High state, transistor MN1 is thus turned on. At this time,the transistor MN2 remains in the OFF state as the output signal ofinvertor INT1 is in the LOW state. P-type transistor MP5 is turned offwhen transistor MN1 is turned on and supply voltage VCC increases, andaccordingly transistors MP1, J3 and MN1 and resistor R3 together form acurrent path to generate the base current I10.

When supply voltage VCC increases and reaches the third thresholdvoltage, such as an under-voltage lock out voltage UVLO of theconversion system, the conversion system enters into the normal stateand the external control signal SHUT transits from the HIGH state to theLOW state. Thus, the output signal provided by inverter INT1 is in aHIGH state to turn off transistor MN1 and to turn on transistor MN2. Thecollector voltage of transistor QN1 and the gate voltage of transistorMN4 are all pulled down to the ground GND as transistor MN2 is turnedon, transistor QN1 and transistor MN4 are thus both turned off, andcorrespondingly, the current path is cut off and the second current I2decreases to zero. On the other hand, charging current Ic decreases tozero accordingly, and current source 300 is turned off. Now, anauxiliary secondary winding of the conversion system operates normallyto supply power to the conversion system, current source 300 is turnedoff to reduce the power consumption of the conversion system.

Transistors J1, J2 and J3 as shown in FIG. 3 are N-type JFETs,transistors MP1, MP2, MP3, MP4 and MP5 are P-type MOSFETs, transistorsMN1, MN2, MN3 and MN4 are N-type MOSFETs, transistor QN1 is an N-typebipolar. It should be known that N-type transistor or P-type transistoras shown may be interchanged for the same functions by combing differentdoping type with different logic state. So the doping types shown inFIG. 3 are only for illustration, N-type transistor may be replaced byP-type transistor and P-type transistor may be replaced by N-typetransistor.

FIG. 4 illustrates a workflow of method 400 of supplying power to anisolation converter with short circuit protection in accordance with anembodiment of the present invention. Method 400 comprises step 401-404.In step 401, rectifying an AC voltage Vac into a line voltage HV. Instep 402, converting the line voltage HV into a second voltage V2 whichis lower than the line voltage HV. In one embodiment, the line voltageHV is converted into the second voltage V2 through a Junction FieldEffect Transistor. In step 403, generating a first current I1 based onthe second voltage V2 and further generating a second current I2 basedon the first current I1. In one embodiment, the step 403 comprisesconverting the second voltage V2 into the first current I1 through aresistor, and then controlling the base-emitter voltage or thebase-collector voltage of a bipolar with the first current I1, andfurther converting the base-emitter voltage or the base-collectorvoltage into the second current I2 through a resistor. In step 404,generating a charging current Ic through a current mirror based on thesecond current I2, wherein charging current Ic is proportional to andlarger than the second current I2. A supply voltage VCC is generated bycharging a capacitor with charge current Ic, wherein by controlling theresistance of the resistor through which the base-emitter voltage or thebase-collector voltage of a bipolar is converted into the second currentI2, the second current I2 is controlled to have a small value whensupply voltage VCC is relatively low, and to have a large value whensupply voltage VCC is relatively high, so as to realize the shortcircuit protection when supply voltage VCC is close to zero on the onehand and to guarantee that supply voltage VCC can increase fast on theother hand. In one embodiment, when supply voltage VCC is smaller than afirst threshold voltage Vth1, the second current I2 is controlled to besmall by controlling a first current regulating resistor and a secondregulating resistor; when supply voltage VCC is larger than a secondthreshold voltage Vth2, second current I2 is controlled to be large byshorting the second current regulating resistor. Wherein the secondthreshold voltage Vth2 may be larger than or equal to the firstthreshold voltage Vth1. As charging current Ic is proportional to thesecond current I2, charging current Ic is controlled to have a firstvalue or Ic<Ith1 when supply voltage VCC is lower than the firstthreshold voltage Vth1 , and charging current Ic is controlled to have asecond value or Ic>lth2 when supply voltage VCC is higher than thesecond threshold voltage Vth2, wherein the second value is higher thanthe first value, namely Ith2>Ith1. In one embodiment, the firstthreshold voltage and the second threshold voltage is the thresholdvoltage that controls the transistor respectively. Method 400 mayfurther comprise controlling the first current, the second current andthe charging current to be zero when supply voltage VCC is higher than athird threshold voltage, wherein the third threshold voltage is higherthan the second threshold voltage. In one embodiment, the first current,the second current and the charging current are controlled to be zerowhen an output voltage of the isolation converter is larger than apredetermined voltage.

From the foregoing, it will be appreciated that specific embodiments ofthe present invention have been described herein for purposes ofillustration, but that various modifications may be made withoutdeviating from the spirit and scope of various embodiments of thepresent invention. Many of the elements of one embodiment may becombined with other embodiments in addition to or in lieu of theelements of the other embodiments. Accordingly, the present invention isnot limited except as by the appended claims.

What is claimed is:
 1. A current source having an input and an output,wherein the current source is configured to receive a line voltage atthe input, and to provide a charging current at the output, and whereinthe charging current is provided to charge a capacitor to generate asupply voltage at the output of the current source, the current sourcecomprising: a conversion circuit configured to convert the line voltageinto a second voltage which is lower than the line voltage; a currentgeneration circuit configured to receive the second voltage and togenerate the charging current; and a control circuit coupled to thecurrent generation circuit, wherein the control circuit is configured toreceive the supply voltage, and to control the current generationcircuit based on the supply voltage; wherein the control circuit isconfigured to control the current generation circuit such that thecharging current is at a first current value when the supply voltage islower than a first threshold voltage and at a second current value whenthe supply voltage is higher than a second threshold voltage, whereinthe first threshold voltage is lower than the second threshold voltage,and the first current value is lower than the second current value;wherein the current generation circuit comprises: a first currentgeneration circuit configured to generate a first current based on thesecond voltage; a second current generation circuit configured togenerate a second current based on the first current, wherein the secondcurrent generation circuit comprises a bipolar junction transistor, andthe second current is regulated by controlling the base-emitter voltageof the bipolar junction transistor with the first current; and a currentmirror having an input and an output, wherein the current mirror isconfigured to receive the second current at the input, and to providethe charging current proportional to the second current at the output;wherein the first current generation circuit comprises: a first JFET; aresistor coupled to the first JFET in series; a first conducting switchcoupled to the resistor in series; a second conducting switch coupled tothe first conducting switch in parallel, wherein the control ends of thefirst conducting switch and of the second conducting switch are coupledto the control circuit; and a current mirror comprising a firsttransistor coupled to the first JFET and a second transistor, whereinwhen either the first conducting switch or the second conducting switchis turned on, the first transistor of the current mirror, the first JFETand the resistor together form a current path to provide a base current,and the second transistor of the current mirror provides the firstcurrent which is proportional to the base current.
 2. The current sourceof claim 1, wherein when the supply voltage is higher than a thirdthreshold voltage, the control circuit is further configured to shutdown the current generation circuit so that the charging current isdecreased to zero, wherein the third threshold voltage is higher thanthe second threshold voltage.
 3. The current source of claim 1, whereinthe first conducting switch is turned off and the second conductingswitch is turned on when the supply voltage is zero; the firstconducting switch is turned on and the second conducting switch isturned off when the supply voltage increases to a threshold voltage; andthe first conducting switch and the second conducting switch are turnedoff when the supply voltage continues to increase to a third thresholdvoltage and an external control signal is in a predetermined state. 4.The current source of claim 1, wherein the first JFET has a pinch-offvoltage.
 5. A current source having an input and an output, wherein thecurrent source is configured to receive a line voltage at the input, andto provide a charging current at the output, and wherein the chargingcurrent is provided to charge a capacitor to generate a supply voltageat the output of the current source, the current source comprising: aconversion circuit configured to convert the line voltage into a secondvoltage which is lower than the line voltage; a current generationcircuit configured to receive the second voltage and to generate thecharging current; and a control circuit coupled to the currentgeneration circuit, wherein the control circuit is configured to receivethe supply voltage, and to control the current generation circuit basedon the supply voltage; wherein the control circuit is configured tocontrol the current generation circuit such that the charging current isat a first current value when the supply voltage is lower than a firstthreshold voltage and at a second current value when the supply voltageis higher than a second threshold voltage, wherein the first thresholdvoltage is lower than the second threshold voltage, and the firstcurrent value is lower than the second current value; wherein thecurrent generation circuit comprises: a first current generation circuitconfigured to generate a first current based on the second voltage; asecond current generation circuit configured to generate a secondcurrent based on the first current, wherein the second currentgeneration circuit comprises a bipolar junction transistor, and thesecond current is regulated by controlling the base-emitter voltage ofthe bipolar junction transistor with the first current; and a currentmirror having an input and an output, wherein the current mirror isconfigured to receive the second current at the input, and to providethe charging current proportional to the second current at the output;wherein the second current generation circuit has an input and anoutput, the input of the second current generation circuit is coupled tothe output of the first current generation circuit, and the secondcurrent generation circuit is configured to provide the second currentat the output, and wherein a collector of the bipolar junctiontransistor is coupled to the input of the second current generationcircuit, and an emitter of the bipolar junction transistor is coupled toa ground, and further wherein the second current generation circuitcomprises: a third transistor coupled between the input of the secondcurrent generation circuit and the ground, wherein a control end of thethird transistor is coupled to the control circuit; a first currentregulating resistor having a first end coupled to a base of the bipolarjunction transistor and a second end; a second current regulatingresistor having a first end coupled to the second end of the firstcurrent regulating resistor, and a second end coupled to the ground; afourth transistor coupled between the first end and the second end ofthe second current regulating resistor, wherein the control end of thefourth transistor is coupled to the control circuit; and a fifthtransistor coupled between the first end of the first current regulatingresistor and the output of the second current generation circuit,wherein the control end of the fifth transistor is coupled to the inputof the second current generation circuit; wherein when the supplyvoltage is higher than the second threshold voltage, the fourthtransistor is turned on to short the second current regulating resistorso that the second current is increased.
 6. The current source of claim5, the control circuit is configured to turn on the third transistor andto turn off the fifth transistor when an external control signal is in apredetermined state.
 7. The current source of claim 5, wherein thecharging current is regulated by adjusting the resistance of the firstcurrent regulating resistor or the resistance of the second currentregulating resistor.
 8. The current source of claim 5, wherein the firstcurrent regulating resistor and the second current regulating resistorare high-sheet poly silicon resistors with a temperature compensationcoefficient.
 9. A current source having an input and an output, whereinthe current source is configured to receive a line voltage at the input,and to provide a charging current at the output, and wherein thecharging current is provided to charge a capacitor to generate a supplyvoltage at the output of the current source, the current sourcecomprising: a conversion circuit configured to convert the line voltageinto a second voltage which is lower than the line voltage; a currentgeneration circuit configured to receive the second voltage and togenerate the charging current; and a control circuit coupled to thecurrent generation circuit, wherein the control circuit is configured toreceive the supply voltage, and to control the current generationcircuit based on the supply voltage; wherein the control circuit isconfigured to control the current generation circuit such that thecharging current is at a first current value when the supply voltage islower than a first threshold voltage and at a second current value whenthe supply voltage is higher than a second threshold voltage, whereinthe first threshold voltage is lower than the second threshold voltage,and the first current value is lower than the second current value;wherein the current generation circuit comprises: a first currentgeneration circuit configured to generate a first current based on thesecond voltage; a second current generation circuit configured togenerate a second current based on the first current, wherein the secondcurrent generation circuit comprises a bipolar junction transistor, andthe second current is regulated by controlling the base-emitter voltageof the bipolar junction transistor with the first current; and a currentmirror having an input and an output, wherein the current mirror isconfigured to receive the second current at the input, and to providethe charging current proportional to the second current at the output;wherein the control circuit comprises: a series circuit having a firstend and a second end and comprising a first JFET, a first resistor and asecond resistor, wherein the first JFET, the first resistor and thesecond resistor are coupled in series, and the series circuit isconfigured to receive the supply voltage at a first end, and is coupledto the ground at a second end, and wherein a first control signal isprovided at a connection node of the first JFET and the first resistorto control the first current generation circuit to generate the firstcurrent when the first current generation circuit starts up, and asecond control signal is provided at a connection node of the firstresistor and the second resistor to control the second current; and aninverter having an input coupled to the first current generationcircuit, and an output coupled to the second current generation circuit,wherein the inverter is configured to receive an external control signaland to turn off the first current generation circuit and the secondcurrent generation circuit when the external control signal is in apredetermined state.
 10. The current source of claim 9, wherein thefirst JFET has a pinch-off voltage.
 11. The current source of claim 9,wherein a threshold voltage for short circuit protection is regulated byadjusting the ratio of the resistance of the first resistor to theresistance of the second resistor.
 12. The current source of claim 1,wherein the conversion circuit comprises a second JFET having a firstend and a second end, the second JFET is configured to receive the linevoltage at the first end, and to provide the second voltage at thesecond end which is coupled to the current generation circuit.
 13. Thecurrent source of claim 12, wherein the second JFET further has acontrol end coupled to a ground.
 14. The current source of claim 5,wherein when the supply voltage is higher than a third thresholdvoltage, the control circuit is further configured to shut down thecurrent generation circuit so that the charging current is decreased tozero, wherein the third threshold voltage is higher than the secondthreshold voltage.
 15. The current source of claim 5, wherein theconversion circuit comprises a JFET having a first end and a second end,the JFET is configured to receive the line voltage at the first end, andto provide the second voltage at the second end which is coupled to thecurrent generation circuit.
 16. The current source of claim 15, whereinthe JFET further has a control end coupled to a ground.
 17. The currentsource of claim 9, wherein when the supply voltage is higher than athird threshold voltage, the control circuit is further configured toshut down the current generation circuit so that the charging current isdecreased to zero, wherein the third threshold voltage is higher thanthe second threshold voltage.
 18. The current source of claim 9, whereinthe conversion circuit comprises a second JFET having a first end and asecond end, the second JFET is configured to receive the line voltage atthe first end, and to provide the second voltage at the second end whichis coupled to the current generation circuit.
 19. The current source ofclaim 18, wherein the second JFET further has a control end coupled to aground.